Is the concept of a perpetual motion machine feasible in the realm of physics?

[yuiop]

Here is how to set up the experiment described in #25:

In Stage 1 the light source is connected to a stored energy source.

1) The electrical energy is converted to electromagnetic radiation energy. (Impossible?)
2) The radiation energy is converted to electrical energy. (Impossible?)
3) The electrical energy powers a motor which starts rotating. (Impossible?)
4) The vertical shaft starts rotating. (Impossible?)
5) The mechanical rotation of the shaft powers a generator. (Impossible?)
6) The battery is disconnected and the electrical output of the generator is connected to the radiation source. (Impossible?)

We have arrived at Stage 2 and essentially the situation in #25.

Which step are you saying is physically impossible?

In stage one the vertical rod is undergoing angular acceleration and observers on the shaft would not say that it is undergoing Born rigid acceleration, but this is of no consequence to the crux of this experiment. At stage 2 when the battery is disconnected, there is no further angular acceleration and after the set up achieves equilibrium, observers on the vertical shaft agree it has Born rigid motion.

Now I agree that 100 percent efficiency is impossible to achieve in principle so we could retain a small power input from a battery at stage 2 to make up for mechanical losses. The important thing is that we should both agree that there is no net production of excess energy (over unity) as result of the gravitational effects and hopefully you will eventually agree that by maintaining the system at an equilibrium of constant angular rotation, that the measurements made by the observers would agree with those given in #25.

Possibly you are saying that 100 percent efficiency is impossible and that is reasonable, so for the sake of argument let us say that the efficiency at each enrgy conversion stage is 99.9%. Let us say further say that at the end of Stage 1 the battery is not disconnected but its output is reduced to a level that maintains constant angular velocity in the system and the energy output of the motor is measured. Would you agree that the energy output from the motor at the top is less than the energy put into light source from the battery? Would you agree that the rpm measurements of the observers at the top and bottom would agree with what I stated in #25 to within a few percent?

 

[bcrowell]

Here is the second paragraph of #26:

BTW, there was a mistake in the way I originally wrote it, but it's not relevant to the point we're discussing here. (Changing coordinates doesn't change the curvature, it changes the field.)

One definition of Born rigid motion is that spatially separated points on the solid body remain stationary and the same distance apart according to accelerating observers at rest with the accelerating body. This excludes your free falling observer.

Just because one way of stating the definition of Born-rigid motion involves a certain type of observer, that doesn't mean that once a Born-rigid body is introduced into a scenario, all other observers become invalid.

There is nothing impossible about the set up in #25 and it could easily be done in any lab. You might disagree with what the scientists might measure, but to say it is impossible to set up the experiment is silly.

Well, of course it can be set up, but the rod won't be Born-rigid. It will flex and deform as it rotates, and this will dissipate heat, causing the transmission of the energy up the shaft to be less than 100% efficient. In a real-life realization of your setup, this general-relativistic effect will be ridiculously small -- much too small to measure compared to effects like frictional heating in the bearings. But the paradox only derives its interest from the idea that it seems like a perpetual-motion machine. The machine is claimed to amplify the input energy by a certain factor 1+ε with each cycle; ε=gh/c² due to the gravitational Doppler shift of the falling photons. In a real-life setup, we'd have ε~10^-16, which is much too small to be practical. When you say that it's "silly" to deny the impossibility of constructing the setup, you're right in the sense that the deviations from rigidity required by relativity would be extremely small, but the whole analysis is an analysis of extremely small effects, in a thought experiment that obviously isn't practical.

 

[yuiop]

Well, of course it can be set up, but the rod won't be Born-rigid. It will flex and deform as it rotates, and this will dissipate heat, causing the transmission of the energy up the shaft to be less than 100% efficient. In a real-life realization of your setup, this general-relativistic effect will be ridiculously small -- much too small to measure compared to effects like frictional heating in the bearings. But the paradox only derives its interest from the idea that it seems like a perpetual-motion machine. The machine is claimed to amplify the input energy by a certain factor 1+ε with each cycle; ε=gh/c² due to the gravitational Doppler shift of the falling photons. In a real-life setup, we'd have ε~10^-16, which is much too small to be practical. When you say that it's "silly" to deny the impossibility of constructing the setup, you're right in the sense that the deviations from rigidity required by relativity would be extremely small, but the whole analysis is an analysis of extremely small effects, in a thought experiment that obviously isn't practical.

I added a paragraph to my last post which you might have missed which partly addresses your concerns here. In the example I gave the redshift was a factor of 2 which far from insignificant. In principle (near a black hole) we can make the redshift factor as large as we desire (eg 100 or 1000) which should be sufficient to overcome mechanical losses. Remember, this is a though experiment and we are talking about what we would measure in principle.

Simplify the experiment even further.

Let us say we have an energised motor low down in a gravitational well. It is connected to a long vertical rod (but no generator or light sources). Let us say the motor reaches an equilibrium terminal rotation speed of 1000 rpm as measured by a local observer. Assume a mechanical and conversion efficiency of greater than 99% (not impossible in principle).

Would you agree that the observer at the top would measure the top of the shaft to rotate at 1 rpm if the redshift factor was 1000?

Would you agree that the observer at the top would measure the both the top and bottom of the shaft to rotate at 1 rpm?

Would you agree that the observer at the bottom would measure the both the top and bottom of the shaft to rotate at 1000 rpm?

(All the above, give or take a small (relative to the redshift factor) efficiency loss).

 

[bcrowell]

BTW, I believe Born-rigidity turns out to completely coordinate-independent, which may help to make it clear that there is no need to worry about which observers we use to judge Born-rigidity. If you take the world-lines of all the particles in the object, they define a congruence: http://en.wikipedia.org/wiki/Congruence_(general_relativity) As described in the WP article, once you have a congruence you can decompose it into parts that describe volume expansion, shear, and vorticity. The volume expansion and the shear have to vanish for Born-rigid motion. Since they're tensors, having them vanish is a coordinate-independent condition. It's possible that I'm messing this up, since I'm not really fluent with this technique, but I think this is right.

 

[guss]

Just for everyone's information, guss duplicated the original post of this thread midway through another thread in the general physics forum: https://www.physicsforums.com/showpost.php?p=3383001&postcount=37 Duplicative discussion of the same problem has been going on in both places. I've closed the other thread. Folks who have been participating in this one may want to pick through the other one for good ideas, and vice versa.

Sorry! I got what I deserved.

By the way, I have been trying to come up with a similar question based on chemical potential energy. I have some ideas of an example, but they are complicated and not as simplified as I would like. I think the answer basically comes out to be that the chemical reaction is dilated in a way that the chemical reaction takes more or less time to complete.

I think a thermodynamics one could be intersting, though I am very unfamiliar with the topic so I wouldn't know where to get started.

 

[bcrowell]

I added a paragraph to my last post which you might have missed which partly addresses your concerns here. In the example I gave the redshift was a factor of 2 which far from insignificant. In principle (near a black hole) we can make the redshift factor as large as we desire (eg 100 or 1000) which should be sufficient to overcome mechanical losses. Remember, this is a though experiment and we are talking about what we would measure in principle.

In this situation, the deformation and heating effects I described in #37 would be very large, and the rod would be vaporized.

Let us say we have an energised motor low down in a gravitational well. It is connected to a long vertical rod (but no generator or light sources). Let us say the motor reaches an equilibrium terminal rotation speed of 1000 rpm as measured by a local observer. Assume a mechanical and conversion efficiency of greater than 99% (not impossible in principle).

Would you agree that the observer at the top would measure the top of the shaft to rotate at 1 rpm if the redshift factor was 1000?

Would you agree that the observer at the top would measure the both the top and bottom of the shaft to rotate at 1 rpm?

Would you agree that the observer at the bottom would measure the both the top and bottom of the shaft to rotate at 1000 rpm?

It would depend completely on the physical properties of the rod. The rod can't rotate Born-rigidly under these conditions.

 

[yuiop]

BTW, I believe Born-rigidity turns out to completely coordinate-independent, which may help to make it clear that there is no need to worry about which observers we use to judge Born-rigidity. If you take the world-lines of all the particles in the object, they define a congruence: http://en.wikipedia.org/wiki/Congruence_(general_relativity) As described in the WP article, once you have a congruence you can decompose it into parts that describe volume expansion, shear, and vorticity. The volume expansion and the shear have to vanish for Born-rigid motion. Since they're tensors, having them vanish is a coordinate-independent condition. It's possible that I'm messing this up, since I'm not really fluent with this technique, but I think this is right.

Consider a rod in flat space that is rotating with constant angular velocity according to an inertial observer. Accelerating observers on the rod agree that it has Born Rigid motion and the relative location of parts of the rod an infinitesimal distance apart do not change over time. To a linearly accelerating observer (not on the rod) the rotating rod appears to accelerate linearly. Where does this leave the Herglotz-Noether theorem?

 

[yuiop]

In this situation, the deformation and heating effects I described in #37 would be very large, and the rod would be vaporized.

Choose a rotation velocity and gravity potential difference where the rod does not vaporize. I really get the impression you do not want to see the "big picture" here.

As I understand it, SR forbids a material with infinite tensile strength, but you seem to be forming your own conjecture/theorem that GR puts a much lower limit on the hypothetical tensile strength of any possible and maybe as yet undiscovered tensile. This conjectured limit of your new theorem is coincidently aligned with the impossibility of measuring relativistic differential time dilation or differential angular rotation of a vertical rod in a gravity field.

Bear in mind that in the Pound–Rebka experiment, the scientists were able to measure time dilation effects over a vertical distance of just 22 metres. I would bet that a few years before the experiment that most people would have thought it impossible to measure gravitational time dilation over a distance of 22 metres in the Earth's gravitational field. Do not under estimate the ingenuity of scientists when it comes to measuring very small effects!

I wonder why you find it difficult to discuss this thought experiment in a "in principle" basis. Is there something about my conclusions that make you uncomfortable? In this forum, we often discuss experiments in an "in principle" basis, such as a rocket ship accelerating to 0.8c in one second or a rocket cruising at 0.99c and objecting to the conclusions on the grounds that is impractical or expensive is just petty and small minded.

 

[Dadface]

What seems to be overlooked here is that despite what is written on the diagram there is no increase of energy when the photons move.It is true that the energy of the photons themselves might change but these changes are accompanied by changes in the gravitational potential energy of the system, the total energy being conserved.

 

[BruceW]

I think the discussion is meant to be general relativistic, which is why I haven't been saying much, since I only know Newtonian gravity.

 

[Q-reeus]

Have come in very late to this thread, and I'm hopping mad! Note that at the end of #6 it is admitted this thread, which has continued on to the current 45 entries, is about a 'perpetual motion machine' gedanken setup. And what's more there have been to date 4 entries from the same individual who shut me down quick smart for posing something also qualifying as 'PMM' gedanken experiment here: https://www.physicsforums.com/showthread.php?t=498821.

To preempt any justification for my contrasting treatment on the grounds of having overtly *claimed* PM whereas the current OP did not, just read the last sentence to my first entry! Yeah , I'm screaming bloody murder over this! Given how swimmingly this thread is chugging along, what exactly is the justification for keeping my thread perma-locked?

 

[I like Serena]

This thread is not about a perpetuum mobile.

It's about understanding how energy conservation works in GR and to improve the understanding of the accepted laws of physics.
The perpetuum mobile drawing is only a means to show in the simplest possible way that the energy must balance.

This is opposed to a machine, in which there must be a flaw somewhere, which may not be evident immediately.
A machine that seems to be intended to show that the laws of physics are wrong.

 

[Q-reeus]

This thread is not about a perpetuum mobile.

It's about understanding how energy conservation works in GR and to improve the understanding of the accepted laws of physics.
The perpetuum mobile drawing is only a means to show in the simplest possible way that the energy must balance.

This is opposed to a machine, in which there must be a flaw somewhere, which may not be evident immediately.
A machine that seems to be intended to show that the laws of physics are wrong.

Before a hasty edit, I got the quoted entry wrong, it should have been #6, not #2 (since corrected), where it reads at the end "...which wouldn't be the right factor to patch up conservation of energy in the perpetual motion machine as originally proposed..." Nobody took issue with that statement. In the case of my thread, where do you find it to be 'complex' in comparison to the current one? Sure it was a multi-scenarios thing, but each scenario is I would suggest of comparable or even lesser complexity than the current one (or variants added along the way!). Besides, the challenge was to find any obvious flaws - an exercise in working out any subtleties involved no different than here surely. If anyone thinks something fundamentally different to that properly applies, I'm all ears!

 

[TrickyDicky]

This is opposed to a machine, in which there must be a flaw somewhere, which may not be evident immediately.
A machine that seems to be intended to show that the laws of physics are wrong.

That kind of gedanken-experiment machines are routinely used as pedagogical means to teach (and learn) physics, like is the case in this thread. Are you opposed to that in principle or only in the case of the locked thread?

 

[TrickyDicky]

I think this is wrong, unless there are holes in the following (admittedly sketchy) argument. The Herglotz-Noether theorem says that in a flat spacetime there are only two types of Born-rigid motion:
1. congruences without rotation
2. uniform translation with uniform rotation
This means that giving rotation to a rigid body causes it to have infinite linear inertia.

Suppose your shaft is in a uniform gravitational field. This can be made into a zero gravitational field by changing to a set of coordinates defined by a free-falling observer. To this observer, the shaft is violating the Herglotz-Noether theorem, so it can't be Born-rigid.


[EDIT] Fixed a mistake above: "can be made into a flat spacetime" -> "can be made into a zero gravitational field"

If the Herglotz-Noether theorem only applies to flat spacetime is of no use in this case. I don't know why you didn't withdraw it even after you acknowledeged the mistake and edited it.

 

[Dadface]

This thread is not about a perpetuum mobile.

It's about understanding how energy conservation works in GR and to improve the understanding of the accepted laws of physics.
The perpetuum mobile drawing is only a means to show in the simplest possible way that the energy must balance.

I wouldn't call it "the simplest possible way",I would call it incomplete and somehow misleading in that most of the ensuing discussions considered the energy changes of the photons only but totally overlooked the attendant gravitational potential energy changes.

 

[TrickyDicky]

What seems to be overlooked here is that despite what is written on the diagram there is no increase of energy when the photons move.It is true that the energy of the photons themselves might change but these changes are accompanied by changes in the gravitational potential energy of the system, the total energy being conserved.

I agree with this view.

I wouldn't call it "the simplest possible way",I would call it incomplete and somehow misleading in that most of the ensuing discussions considered the energy changes of the photons only but totally overlooked the attendant gravitational potential energy changes.

It is not totally overlooked IMO, I think that yuiop take is in line with the global conservation you bring up. This is included in the gravitational redshift-time dilation with which he explains that there is no real energy gain.

 

[Dale]

. And what's more there have been to date 4 entries from the same individual who shut me down quick smart for posing something also qualifying as 'PMM' gedanken experiment here: https://www.physicsforums.com/showthread.php?t=498821.

Let's make it five.

Seriously, if you think this is a PMM discussion then report it, you should always report content you feel is inappropriate regardless of who is posting in the thread. I got a different tone from this one and yours, but feel free to call me a hypocrite directly and by name if it will make you feel better.

 

[Q-reeus]

...Seriously, if you think this is a PMM discussion then report it, you should always report content you feel is inappropriate regardless of who is posting in the thread...

But I have no issue with the OP's presentation - why would I report him when I had posted something along the same lines? My report would be about selective treatment, not innapropriate topic. And that gets down to use and interpretation of the PF guidlines re 'perpetual motion'. To quote the appropriate PF Rules section here: https://www.physicsforums.com/showthread.php?t=414380 , under 'Overly Speculative Posts':
"One of the main goals of PF is to help students learn the current status of physics as practiced by the scientific community; accordingly, Physicsforums.com strives to maintain high standards of academic integrity. There are many open questions in physics, and we welcome discussion on those subjects provided the discussion remains intellectually sound. It is against our Posting Guidelines to discuss, in the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion. Non-mainstream or personal theories will be deleted. Unfounded challenges of mainstream science and overt crackpottery will not be tolerated anywhere on the site. Linking to obviously "crank" or "crackpot" sites is prohibited."

That passage has been significantly edited since my thread was locked, when the above section contained a rather brief and generally worded warning about discussing PMM's and 'free energy'. Trouble is there was no context provided. As pointed out in PM correspondence following my lockout, it's one thing to be linking to and quoting as authority some 'free energy' charlatan; quite another to present a properly reasoned scenario that at least *seems* to violate some established law. And imho that distinction should be acknowledged, have a proper formal policy attached, and then fairly adhered to. As you and many here are well aware there is a fairly regular stream of OP's making claims along PMM lines, many times with hardly any proper forethought. Lockout is a rarity. Mostly there is allowed reasoned discussion as pedagogical exercise. Entry #49 put it quite well.

I got a different tone from this one and yours, but feel free to call me a hypocrite directly and by name if it will make you feel better.

I have already pointed to the passage in #6 where it was acknowledged the OP had posed a nominal PMM scenario. Your own second entry began immediately following that! I'm not going to call you a hypocrite because just maybe you missed that passage, or whatever else you may have been thinking I don't know. Now if it gets down to a matter of different tone deciding things, that raises the issue of style vs substance. Suppose the OP had added the words "this looks like PM to me" or similar, but left everything else the same; Would you have pushed the lockout button on that basis? One should not have to tread on egg-shells imo. If the substance is a legitimate topic for discussion - and clearly it has so been judged here, why should the OP's opinion (perhaps just a matter of being rhetorical or dramatic) make a dramatic difference in outcome? Substance, or style?

 

[danR]

I must complain that everyone is changing Guss' 'original' specification to an interesting conundrum they like but gets way too technical for me, and which doesn't come close enough to the original to answer it in an intuitive and transparent way.

Guss has specified a photon emitter. I will deal with it one photon at a time.

His assembly of gears is an assembly of levers and fulcra, and we can simplify that to the instantaneous case of a vertical lever and fulcrum. When we're finished we can re-assemble it as multiple gears again, because what is happening amounts to the same.

I will assume a (theoretically possible?) perfect momentum absorber as his energy converter at the bottom.

One photon bangs into the absorber at the bottom of the lever and sends a transverse wave up the lever. This is the crux of the matter: the impulse is mediated by the bonding force of the covalent shared electron bonds of the metal of the lever, and the bonding force is electromagnetic, whose associated particle is the photon.

I humbly submit that the wavelength of the photons involved at bottom and top of the lever are correlated with the wavelength of the photon moving down from top to bottom, and the net energy gain is zero, and a fortiori, any mechanical assemblage that moves energy from bottom to top is going to have the same symmetrical configuration.

 

[Q-reeus]

Here is a simplified version of the original problem:

Assuming 100% efficiency and assuming the light is blue shifted by a factor of 2 on its way down, then if the motor at the bottom is rotating at 1000 rpm measured locally, the generator at the top would rotate at a rate of 500 rpm measured locally. This would cancel out the apparent energy gain of the light traveling downwards. The observer at the top would see both the motor and generator as turning at a rate of 500 rpm and the observer at the bottom would see both the motor and generator as turning at a rate of 1000 rpm. If this was not the case, the connecting shaft would eventually break.

If the observer at the top sent a synchronisation signal down at a rate of once per second and the observer at the bottom synchronised his clock with the signal from the top, both observers would agree that the top and bottom of the connecting rod is turning at 500 rpm and that the apparent increase in energy of the falling photons is just an artefact of the difference in clock rates of the unsynchronised clocks and that there is no actual energy gain.

Well since I'm here, might as well add my 2 cents. yuiop I believe you are not quite right here, but no-one else has picked it up so here goes. Take a trip down memory lane - here to be exact: https://www.physicsforums.com/showthread.php?p=3026387.
We both got it initially wrong, ich in #5 gave the correct answer. Power top vs bottom goes as the square of redshift - not only are the individual photons blueshifted bottom re top, but their rate of arrival also. And thus vice versa when it comes to translating mechanical power from bottom to top in your setup. The redshifted motor shaft speed is correct, but since power = angular velocity x torque, one must find the torque redshifted equally. Since by the Schwarzschild coords lateral dimensions and thus shaft radius is invariant wrt potential, this boils down to the azimuthal forces acting on the motor rotor must be redshifted. In another thread I used the example of a horizontally disposed dipole undergoing partial collapse, giving off redshifted radiation in the process. As the initial moment arm is an invariant there also, redshifted emission of radiation must coincide with a redshifted force of attraction between charges. This is consistent then with then a (redshift/blueshift)² relation between power measured at the two different locales. Of course one still has to be comfortable with the motor-shaft-generator setup being an entirely general representation of mechanical power transport up against gravity. Do we agree?

 

[guss]

I must complain that everyone is changing Guss' 'original' specification to an interesting conundrum they like but gets way too technical for me, and which doesn't come close enough to the original to answer it in an intuitive and transparent way.

Guss has specified a photon emitter. I will deal with it one photon at a time.

His assembly of gears is an assembly of levers and fulcra, and we can simplify that to the instantaneous case of a vertical lever and fulcrum. When we're finished we can re-assemble it as multiple gears again, because what is happening amounts to the same.

I will assume a (theoretically possible?) perfect momentum absorber as his energy converter at the bottom.

One photon bangs into the absorber at the bottom of the lever and sends a transverse wave up the lever. This is the crux of the matter: the impulse is mediated by the bonding force of the covalent shared electron bonds of the metal of the lever, and the bonding force is electromagnetic, whose associated particle is the photon.

I humbly submit that the wavelength of the photons involved at bottom and top of the lever are correlated with the wavelength of the photon moving down from top to bottom, and the net energy gain is zero, and a fortiori, any mechanical assemblage that moves energy from bottom to top is going to have the same symmetrical configuration.

I kind of agree, I still think they are talking about my problem, but I can't follow the discussion.

I *think* the original problem has been solved, but there is still disagreement. And yes, your simplified way of thinking about it is better.

By the way, how is the energy of te photons not increasing? They are being pulled down by gravity, thus their mass should be increasing, no? If not, that should be beside the point (not that I am not interested in the fact that I'm wrong about the photons).

The point is to evaluate the movement of mechanical energy against gravity.

 

[Dale]

Now if it gets down to a matter of different tone deciding things, that raises the issue of style vs substance. ... why should the OP's opinion (perhaps just a matter of being rhetorical or dramatic) make a dramatic difference in outcome? Substance, or style?

Tone does make a difference to me. Feel free to be less influenced by style than I am in deciding which posts you choose to report or not.

I don't feel bad about reporting your previous thread, and although I didn't report this one I wouldn't have disagreed if someone else did.

 

[I like Serena]

I kind of agree, I still think they are talking about my problem, but I can't follow the discussion.

I *think* the original problem has been solved, but there is still disagreement. And yes, your simplified way of thinking about it is better.

By the way, how is the energy of te photons not increasing? They are being pulled down by gravity, thus their mass should be increasing, no? If not, that should be beside the point (not that I am not interested in the fact that I'm wrong about the photons).

The point is to evaluate the movement of mechanical energy against gravity.

I didn't follow most of the discussion either, but here's what I got out of it (simplified thinking style ).

The mechanical contraption is indeed subject to GR.
At the bottom the gravitational potential is lower, meaning more time dilation.

The difference in time dilation is responsible for the apparent loss of energy when going up.

And moreover, this same time dilation is also responsible for the blue shift of the photons going down.

 

[danR]

I kind of agree, I still think they are talking about my problem, but I can't follow the discussion.

I *think* the original problem has been solved, but there is still disagreement. And yes, your simplified way of thinking about it is better.

By the way, how is the energy of te photons not increasing? They are being pulled down by gravity, thus their mass should be increasing, no? If not, that should be beside the point (not that I am not interested in the fact that I'm wrong about the photons).

The point is to evaluate the movement of mechanical energy against gravity.

I agree that your downward photons increase energy. Perhaps I gave another muddled explanation. But transmitting energy through a mechanical means will go through that little song-and-dance I gave that I'd hate to repeat. There is no simple way of explaining it; it would require a new post giving all the details by an expert.

The glue particle of electromagnetism is the photon, it is the exchange particle of the electromagetic force. No wave, phonon, impulse, twist, etc. can travel without these mediating photons acting at every step. With each step up the gravity gradient, the exchange-photon will have tad lower energy. So we're looking at single photons coming down and getting more energetic, and domino-photons, if you will, going up the gears getting weaker. As each exchange photon is emitted and absorbed a bit higher, it is a bit weaker. (They may be 'virtual' photons, for all I know. Perhaps they don't obey GR rules.)

So when you get to the top, you're right back where you started. If you rob the system of energy, then you have the classic, non-functioning, perpetual motion machine. There will be a net loss. The thing will run out of fuel.

There would be a similar problem if you just had a big vertical circuit of wire. The deBroglie waves coming down you might think would have more energy (voltage) at the bottom, but they would have less at the top.

 

[Dadface]

There seems to be an assumption in this thread that the downcoming photons are adding energy to the system.They are not.Let me reiterate the points I made earlier:

The total energy of the system remains constant and there are no energy increases when the photons move down or energy decreases when the photons move up.

When photons are gaining energy by blue shifting the system is losing potential energy and when they are losing energy by red shifting the system is gaining potential energy.

 

[Dale]

As I understand it, SR forbids a material with infinite tensile strength, but you seem to be forming your own conjecture/theorem that GR puts a much lower limit on the hypothetical tensile strength of any possible and maybe as yet undiscovered tensile.

The conjecture which puts a much lower limit on the tensile strength of a material is called the "weak energy condition". Basically it says that the energy density in every frame must be non negative everywhere. Tension in one frame reduces the energy density in other frames, so this places a finite limit on the tension.

 

[Q-reeus]

Tone does make a difference to me. Feel free to be less influenced by style than I am in deciding which posts you choose to report or not.

I don't feel bad about reporting your previous thread, and although I didn't report this one I wouldn't have disagreed if someone else did.

If style or tone or tenor is actually the difference, how about giving me a straight answer to the following. Suppose I 'repackage' that closed thread and post it anew, thoroughly cleansed of any trigger words like 'free energy', 'perpetual motion' etc, or assertive statements claiming conservation laws might be under a cloud. Given your participation in this thread, would you move to block me again? Yes or no please, and with a reason.

 

[danR]

There seems to be an assumption in this thread that the downcoming photons are adding energy to the system.They are not.Let me reiterate the points I made earlier:

The total energy of the system remains constant and there are no energy increases when the photons move down or energy decreases when the photons move up.

When photons are gaining energy by blue shifting the system is losing potential energy and when they are losing energy by red shifting the system is gaining potential energy.

I can see that, but I can't see the net zero gain of the whole system until the whole energy cycle is parsed according to its components. The downward photons are more energetic, and their number is not diminished. The loss of potential energy is not found in basic high-school physics bookkeeping until the potential energy is regained by the arrival of return red-shifted photons.

 

[guss]

There seems to be an assumption in this thread that the downcoming photons are adding energy to the system.They are not.Let me reiterate the points I made earlier:

The total energy of the system remains constant and there are no energy increases when the photons move down or energy decreases when the photons move up.

When photons are gaining energy by blue shifting the system is losing potential energy and when they are losing energy by red shifting the system is gaining potential energy.

But why? Is it simply because the standard E = mgh doesn't apply because the rest mass of a photon is 0? So that E = mgh always uses rest mass and not relativistic mass?

 

[Dale]

how about giving me a straight answer to the following ... would you move to block me again? Yes or no please, and with a reason.

I can't give you a definite reaction to a hypothetical post. But hypothetically if there were nothing objectionable then even if I did the moderators would ignore me.

 

[Q-reeus]

I can't give you a definite reaction to a hypothetical post. But hypothetically if there were nothing objectionable then even if I did the moderators would ignore me.

OK let's leave it at that for now then, and maybe see what transpires down the track a bit.

 

[Dadface]

I can see that, but I can't see the net zero gain of the whole system until the whole energy cycle is parsed according to its components. The downward photons are more energetic, and their number is not diminished. The loss of potential energy is not found in basic high-school physics bookkeeping until the potential energy is regained by the arrival of return red-shifted photons.

High school students study energy changes and would know,for example,that when an object is falling freely there is a conversion of PE to KE.The part of the event where the object is falling is analogous to the part of the event described by guss where the photons are moving down.

 

[Dadface]

But why? Is it simply because the standard E = mgh doesn't apply because the rest mass of a photon is 0? So that E = mgh always uses rest mass and not relativistic mass?

For a photon E=hf=mc^2 where m= the effective mass of the photon.

 

[yuiop]

High school students study energy changes and would know,for example,that when an object is falling freely there is a conversion of PE to KE.The part of the event where the object is falling is analogous to the part of the event described by guss where the photons are moving down.

In Newtonian physics, the potential energy is hypothetically stored in the gravitational field, but in GR it is not necessarily that simple and as far as I know energy has to be more directly accounted for in GR. In some ways in Newtonian physics, potential energy is a bookeeping exercise to keep the energy balance straight. In GR any energy has a gravitational effect and we have to state where that energy is, AFAIK. As you can probably tell, I am really not sure of the absolute answers here, so maybe some of the more enlightened could shed some light on this.